This invention relates to opto-electronic scale-reading apparatus. International Publication No. WO86/03833 published July 3rd, 1986 discloses an opto-electronic scale-reading apparatus comprising a scale defined by spaced lines, a read head, the read head and the scale being movable one relative to the other in the direction of spacing of the lines, the read head comprising projecting means for projecting light on to the scale along a first path, the scale co-operating to pass on the light from the projecting means along a second path, the light so passed on having a pattern corresponding to the spacing of the lines, receiving means for receiving the light as passed on by the scale along said second path, and spatial filter means arranged in said second path for determining that, regarding the pattern passed on from the scale, substantially only lines of a given periodicity are transmitted by the spatial filter means.